1.1 Field of the Invention
The present invention pertains generally to dynamic brakes, and more particularly, to dynamic brakes that permit rapid stopping of alternating current (AC) motors through the use of direct current (DC) injection to create a stationary, braking, magnetic field within the motor.
1.2 Description of the Prior Art
AC motors are widely used in industry. For example, the large machines used in most wood processing plants, e.g., chippers, band saws and planers, are almost exclusively driven by AC motors. In industrial plants where AC motors are used and relied upon, an inoperable AC motor can bring an entire production line to a full stop. Accordingly, controlling and maximizing the productive time of each AC motor is important. One way to enhance the productive time available of an AC motor is through proper control of the motor's braking cycle.
Generally, when the power to an AC motor is cut off, the motor does not come to an immediate stop, but rather gradually "coasts" to a stop. For large motors with large loads this "coasting" period can be quite long (20-40 minutes for massive loads) resulting in periods when the motors are unavailable for useful work. In many cases (e.g., when changing motor loads) these "coasting" periods result in gaps of time when both the motor and the motor's operator are unproductive.
In addition to the productivity problems caused by coasting, allowing a motor to gradually coast to a stop can be quite hazardous. For example, a silently coasting machine such as a radial arm saw, disc sander or band saw can cause tremendous injury to both man and machine.
In an effort to reduce the unproductive and hazardous coasting periods associated with AC motors, mechanical or friction brakes are often used to decrease the time required to bring a running AC motor to a stop. Such brakes rely on the frictional force created between a mechanical brake pad and a rotating part of the motor. Because the brake pad often wears away after several braking cycles, mechanical brakes require significant adjustment, repair and maintenance (e.g., brake pad replacement). A further disadvantage with many mechanical brakes is that the brake pads for which the brakes were designed are manufactured from asbestos or asbestos substitutes--potential cancer-contributing materials most industries are hesitant to use.
In an effort to overcome the disadvantages associated with mechanical brakes, frictionless electronic brake assemblies were developed. Such frictionless electronic brake assemblies are often referred to as "electronic dynamic brakes."
The basic operation of a typical electronic dynamic brake assembly is illustrated in FIG. 1. As illustrated in FIG. 1, an electronic dynamic brake 10 is coupled via electrical connections 12, 12', 14 and 14' to the power line inputs and to a three-phase AC motor 20. Sense leads 16 and 16' detect motor contractor opening to initiate the brake cycle and are used to sense whether power is being applied to the motor. Connected across the power line inputs is a motor starter 30 that includes three contacts for controlling the power line inputs to the motor. When the START button on the motor starter is activated, the three contacts are closed, coupling the motor inputs to the power line inputs. When the STOP button of the motor starter is activated, the three contacts are opened and the three motor inputs are disconnected from the power lines.
The sense leads 16 and 16' of the electronic brake 10 monitor the power applied to the motor 20. When sense leads 16 and 16' sense that power is being supplied across the motor starter contacts to the motor, an indication that the motor is RUNNING is provided to the brake assembly. If, once the motor is running, the sense leads 16 and 16' sense that the power supplied to the motor has been cut off (e.g., by activation of the STOP button) the control leads provide an indication to the dynamic brake that a braking cycle should be instigated. The dynamic brake assembly will use the power it receives from the power lines to generate direct ("DC") current. This direct current is then injected, through motor input lines (a) and (c) into the stator of the AC motor 20. The injection of the DC current creates a stationary magnetic field within the motor. This stationary magnetic field forces the poles of the rotor field to align with the stationary poles of the stator, quickly brings the motor to a stop without mechanical friction. Because the braking cycle of the dynamic brake discussed above is initiated by activating the STOP button of the motor and is controlled by the motor starter, this type of brake assembly is commonly referred to as a "slave brake."
Because it could damage both the motor and the dynamic brake if a braking cycle were initiated when three-phase power is being supplied to the motor, most electronic dynamic brake assemblies are used in conjunction with a separate, electrical interlock system. An electrical interlock system is illustrated as part of element 10 in FIG. 1. Basically, the purpose of the electrical interlock is to ensure that the motor cannot be energized by the starter contacts during a braking cycle. During normal motor running operation, the interlock circuit is closed and power from the power lines is applied to the motor starter. During a braking cycle, the interlock opens up, thus "locking out" the motor starter and ensuring that power cannot pass through the motor starter to the motor. The installation and operation of electrical interlocks is understood by those skilled in the art and will not be further addressed herein.
When using an electronic dynamic brake, the length of the braking cycle (i.e., the time necessary to bring the motor to a complete stop) will vary depending on the magnitude of the DC braking current applied to the motor, the size and type of motor, and the size and type of load attached to the motor. In many prior art brake assemblies, a timer is used to ensure that the DC braking current is applied to the motor for a sufficient time period to bring the motor to a stop.
When a timer is used with a electronic dynamic brake, the brake is initially set so that when a braking cycle is initiated, the brake will apply DC current to the motor for a preselected time period. The time period is usually set--through trial and error--to be of sufficient length to bring the motor to a complete stop. Because variables such as the line voltage, temperature of the motor and slight load changes can affect the time required to brake a motor, the braking time period is usually selected to be longer than the maximum expected stopping time.
One disadvantage of "timed" electronic dynamic brakes is that the set time period (the maximum braking time) is often longer than the actual time period required to stop the motor. Accordingly, with timed dynamic drakes, there are often periods--referred to as dead time--when the motor has come to a stop, but the dynamic brake assembly continues to inject DC current into the motor. Excessive dead time frequently occurs when the load applied to the motor is smaller than normal. Such dead time periods, like the coasting periods discussed above, render the motor unproductive. Additionally, such periods waste power as DC current is unnecessarily being supplied to a stopped motor.
A further disadvantage of timed electronic dynamic brakes is that the preselected time period is generally optimized for a single motor and a single load. Accordingly, if the load changes, or the brake assembly is moved to a different motor, the brake assembly must be tested and reset to properly brake the new motor or load. Such resetting periods are inefficient in that during such periods it is difficult to efficiently use the brake assembly, the new motor, the new load or the technician who is responsible for resetting the brake. A still further disadvantage with timed electronic dynamic brakes is if, for some reason, the preselected time period is too short, the brake will stop injecting DC current into the motor before the motor is stopped, resulting in a "coasting" period like the ones discussed above.
In an effort to overcome the disadvantages associated with timed dynamic brakes, some in the prior art began to use "stop sensors" or "zero speed sensors" in conjunction with typical electronic dynamic brakes. These prior art zero speed sensors would detect the rotation of a three-phase motor by sensing the voltage at the motor input terminals. For example, as illustrated in FIG. 1, a lead 17 could be used to serve the voltage at a motor input terminal.
One type of prior art zero-speed detector is illustrated in FIGS. 2A and 2B. FIG. 2A is a partial schematic diagram of one prior art zero-speed sensor. The sensor generally comprises a differential sense amplifier that receives as its inputs the signals T1 and T2, from two input terminals of an AC motor. The differential amplifier 53 receives the two signals and produces an output signal proportional to their difference. The differential output signal is then applied to a low pass filter 54 and the output of the low pass filter is applied to a 60 Hz. notch filter 55. The notch filter is used to filter out any voltage waveforms caused by the 60 Hz. AC power typically applied through the power input lines to AC motors. The output from the notch filter 55 is then applied to four sample and hold circuits 56. The sample and hold circuits are configured such that two of the circuits are clocked every 60 Hz. cycle. The outputs form the sample and hold circuits 56 are then applied to additional circuitry 57 (not illustrated in detail) that produce a ZERO SPEED signal when certain conditions are met.
During a braking cycle the rotational speed of the motor being braked will normally be constantly decreasing. Because the rotational speed of the motor is constantly changing, so too are the voltages generated by the motor at its input terminals. When the motor has come to a complete stop, the voltage at the output terminals will be essentially constant. Accordingly, by monitoring the change in the voltages at the motor input terminals, and determining when the voltages cease to change, it is possible to sense when the motor has come to a stop.
The prior art zero-speed circuit of FIG. 2A generally operates as follow: First, two times during each 60 Hz. cycle, samples are taken of the output of the 60 Hz. notch filter 55. This is illustrated in FIG. 2B, where samples are shown being taken at points A and B. The two samples are then stored in two of the sample and hold circuits (A, B) 56. During the immediately following 60 Hz. cycle, two more samples are taken (A', B') and stored in sample and hold circuits (A', B') 56. The most recent samples are then compared to the previously taken two samples by circuitry 57. If the sample for A does not match the sample for A' (or if B does not match B') then there is no zero speed. If, however the pairs of samples match, then there is a chance that the motor is at zero-speed. In most prior art devices, the samples must match for a sufficient number of cycles, e.g., 40-100, before a zero-speed signal is generated. Accordingly, circuitry 57 monitors the outputs of the sample and hold circuits 56 and generates a ZERO-SPEED signal whenever the sample and hold pairs match for the preselected number of times.
One problem with prior art zero-speed detectors is that for most motors, there are periods during a braking cycle where portions of the output voltage at the terminals do not change, even though the motor is rotating. These periods are referred to as "dead spots." If the zero-speed sensor happens to take samples of the voltages from these "dead spots", it can be fooled into reporting that a motor is stopped when, in fact, the motor is still rotating. In order to avoid false zero speed detections, manual adjustments are required in many prior art zero speed detectors. Common adjustments included altering the time during the 60 Hz. cycle when the two samples are taken, and increasing the number of matches that must be detected before a ZERO-SPEED signal is generated.
The location of the "dead spots," discussed above, varies from motor to motor and load to load. Furthermore, the location of dead spots for the same motor and load can vary as the motor wears, the load changes, the line voltage varies or the temperature of the motor or load changes. Accordingly, it is quite difficult to set prior art zero-detectors for a particular motor/load/temperature range combination and--once the brake is set--it is difficult to move the brake to another motor/load/temperature combination without extensive resetting. In many instances, the "worst-case" scenario is used to set the zero-detector. In other words, the zero-speed detector is set to produce a ZERO-SPEED signal only after the number of matches indicated is such that it exceeds the worst case dead spot. Accordingly, in many cases, prior art zero-speed detectors require more matches prior to the generation of a zero-speed signal than are actually necessary.
Another problem with prior art zero-speed detectors is that they are operable at a sub-optimum time in the firing procedure. In most prior art zero-speed detectors, the zero speed sensing occurs after a delay and after a SCR firing. Thus there is a three-step procedure: (1) delay; (2) fire; and (3) zero-speed sensing. This is believed to provide sub-optimum results as the firing of the SCR immediately before the zero-speed sense is believed to negatively impact the zero-speed sensing.
The prior art zero-speed detectors are used with a variety of electronic brake assemblies. Generally, there are three different types of prior art brake assemblies:
(1) Slave Brakes--Discussed above, where the activation of the braking cycle is dependent on the position of the motor contacts in the motor starter (i.e., the motor contacts control the brake assembly); PA1 (2) Pre-Stop Brakes--where the brake brings the motor to a complete stop before each start; and PA1 (3) Holding Brakes--where the brake, when activated, continuously supplies DC current to the motor to prevent it from rotating (when the START button is activated, the brake releases).
As discussed above, slave brakes are generally used to simply bring a motor to a stop when the STOP button is activated.
Pre-stop brakes are used in situations (such as in wind tunnels with fans) where there is chance that, prior to starting, the shaft of the motor will be rotating opposite to the desired direction of rotation. Such opposite rotation is referred to as "windmilling." Sever damage can occur to the motor and the load if it is started while the motor is windmilling. Pre-stop brakes overcome the windmilling problem by sensing when the START button has been activated, bringing the motor to a complete stop, and then releasing the brake and allowing the motor contacts to close, starting the motor. Unlike slave brakes, where the status of the motor contacts determines the status of the brake, in a pre-stop brake circuitry within the pre-stop brake assembly controls the status of the contacts in the motor starter.
Holding brakes are used when it is necessary to ensure that the motor does not rotate after it has been stopped. Holding brakes, when activated, bring the motor to a stop and then continuously apply DC current to the motor to ensure that the shaft does not rotate. Holding brakes are often used with dangerous equipment when it is important to ensure that the equipment does not move when not in operation or where a positive hold on the motor shaft must be maintained to prevent backward coasting (e.g., inclined conveyors).
In the prior art, each type of brake (slave, pre-stop and holding) requires a discrete and separate brake assembly. Accordingly, if one wants a holding brake, one purchases and installs a holding brake assembly. If one wants a pre-stop brake, a pre-stop brake must be installed. One difficulty with these separate prior art devices is that it is often difficult to accurately determine what type of brake is needed for a particular application. For example, one may initially determine that a slave brake is needed but, after observing wind and rotational effects, realize that a pre-stop brake is required. With prior art brake assemblies, one is forced to remove the installed slave brake and replace it with a different pre-stop unit. Such replacement of installed brake assemblies results in a loss of both the time and cost of replacement as well as the extra time the motor is unproductive.